The Varian Mat GD150 for argon analyses in
connection with K-Ar dating
Autor(en):
Purdy, John W.
Objekttyp:
Article
Zeitschrift:
Eclogae Geologicae Helvetiae
Band (Jahr): 65 (1972)
Heft 2
PDF erstellt am:
15.06.2017
Persistenter Link: http://doi.org/10.5169/seals-164095
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Eclogae geol. Helv.
Vol. 65/2
Pages 317-320
1
figure in the text
and table
1
Basle, Aug. 1972
The Varian Mat GDI50 for Argon Analyses in Connection
with K-Ar Dating
By John W. Purdy1)
Mineralogisch-Petrographisches Institut, Universität Bern, CH-3012 Bern, Switzerland
ABSTRACT
A Varian Mat GD 150, a relatively inexpensive gas source mass spectrometer is discussed from
the viewpoint of its suitability for argon analyses in connection with K-Ar dating. It is shown that
the instrument is satisfactory for this type of research.
Introduction
In 1964 Farrar et al. reported on their successful adaptation of the Associated
Electrical Industries Limited MS 10 mass spectrometer for argon analyses. Their
report clearly demonstrated that it was possible to establish a K-Ar dating laboratory
capable of measuring precise geologic "ages" employing a relatively inexpensive gas
source mass spectrometer. Since the above report other commercial instruments have
become available, including the Varian Mat GD 150. It is the purpose of the present
communication to describe the Varian Mat GD 150 as it has been set up in our
laboratory with particular emphasis to the usage of this instrument for the measure¬
ment of argon extracted from rocks and minerals.
Description of the instrument
The mass spectrometer is an all metal, bakeable, 180° analyser having a 2-cm
and 5-cm radius of curvature collector as well as a total ion collector. The sample is
admitted directly into the ionization box; ions being produced in a Nier (1947) type
electron impact source. A particular feature of the ion source, not always found on
small commercial instruments, is its electric lens system consisting of draw out plates,
lenses, and a focusing plate. The potential on each of these plates and half-plates is
variable within certain limits giving the operator some flexibility in adjusting the ion
beam for optimum intensity, peak shape and resolution. A permanent magnet of
0.385 Wb M~2 deflects the ions through 180°. Mass selection is achieved by varying
the ion accelerating voltage either manually or automatically (electronically).
^Present Address: Sub-department of Geophysics, Oliver Lodge Laboratory, Oxford Street,
Box 147, Liverpool L69 3BX, United Kingdom.
P. O.
J. W. Purdy
318
The amplifiers (two), potential sources for accelerating voltage and ion optics are
solid state apart from a high voltage tube and two electrometer tubes. The noise level
of the amplifiers corresponds to an ion current of 2.10~15 A. On our instrument the
5-cm collector port has been adapted to the head of a Cary vibrating reed electrometer.
The pumping system is a standard high vacuum one consisting of a liquid nitrogen
cold trap, two mercury diffusion pumps and a rotary pump.
Performance of the instrument
2 A) is
Baking the mass spectrometer overnight at 250 °C (filament current
sufficient to reduce the final pressure in the analyser to the order of x Ì09 Torr.
After baking, running the filament for a few hours at a higher trap current (225 u.A)
than normally used (80 ¡xA) results in the dynamic background in 34—43 mass region
being reduced to an undetectable level (< 2.10~15A). Operated statically (80 ¡¿A trap
current) there is a slowly rising ion current at mass 40 (2.10-15 A minX and 38
(LIO-15 A min1). There is no detectable ion current (< 1.10~15 A) at mass position
36 even when the analyser tube is closed from the pumps for more than one hour.
After preliminary checks with various combinations of source exit slits (0.06, 0.15,
0.25 mm) and collector slits (0.35, 0.50 mm), an exit slit of 0.15 mm and a collector
slit of 0.50 mm was selected as offering the best compromise of sensitivity, peak shape
and resolution. The static sensitivity for argon is 6. IO-5 A cm ~3STP (80 fxA trap
4.3 V). An indication of the peak shape and resolution
current, ion repeller voltage
38
40
and
for the argon 36,
masses can be seen in Figure 1. This mass spectrum was
recorded by feeding the output from the amplifier supplied with the instrument to a
10-inch strip chart recorder. The argon sample was a mixture of a few-hundred-mg
of an Alpine mica, 1.5 x 10 6cm3 STP of Ar38 "tracer" and atmospheric contamination.
1
X0-03
n
36
38
40
Fig.
1.
Recording of typical argon measurement.
Mass spectrometer for
K-Ar dating
319
The 40 and 38 masses contributed negligibly to each other. The contribution of the
38-ion current to the 36-ion current is < 6.10 5 times the height of the 38-ion current.
The mass spectrometer does exhibit a "memory" effect which, for routine sample
analysis can be minimized or virtually eliminated in the usual fashion by choosing
the amount of sample so the isotopie composition of the argon admitted to the mass
spectrometer is always similar. In those cases where a memory effect is noted a typical
variation of the (40/38) ratio is (0.05-0.1 % min * for samples having a (40/38) ratio
between 0.1 and 10.
Farrar et al. (1964) reported the absence of a memory effect for the MS 10. This
was attributed to the low accelerating voltage 100 V) for argon isotopes and consequent
smaller likelyhood of embedding argon ions into the walls of the spectrometer tube
which, in turn, may be readmitted to the analyser tube during a later analysis. More
recently, Rex and Dodson (1970) have described their experience with a MS 10
employing a more powerful (0.41 Wb M~2) magnet than was used by Farrar et al.
(0.183 Wb M~2). Using the stronger magnet Rex and Dodson (1970) noted a memory
effect. They reported a variation of 0.05% min-1 for the (40/38) ratio of a "spiked"
sample analysed after the mass spectrometer had been used for running atmospheric
argon samples. This (40/38) variation is similar to the one noted above for the GD150.
Apart from the above mentioned "memory" effect, there is no indication of other
variations in the measured isotopie ratios for purified argon samples. The measured
ratios are, however, affected by the setting of the ion repeller voltage, the latter being
variable between 0 and 10 V positive with respect to the ionization box. For our
instrument, a setting of 4.3 V gives a minimum change in the measured ratios with
a change in ion repeller voltage. A change of 0.3V at 4.3 V produces a change in the
measured ratios of less than 0.05%.
With an ion repeller voltage of 4.3 V and a trap current of 80 ¡J.A, the value
measured for the (40/36) ratio of atmospheric argon is 292.4. This value has been
repeated over a period of several months with a standard error of 0.4%. The value
292.4 for the (40/36) ratio is an average of values measured by leaking a small fraction
of argon into the analyser tube from a large reservoir, necessitating multiplying the
measured ratios by | 40/36; and of values measured by equilibrating a small amount
of argon between the analyser and inlet system. Within the stated precision, there
is no difference between the (40/36) ratio values measured in these two ways.
Comparing the value 292.4 with the value 295.5 determined by Nier (1947) indicates
the mass discrimination of the instrument with the above source conditions.
In the Table are given the results of argon concentration determinations of a
number of interlaboratory standards. The argon was extracted in vacuo using radio
frequency induction heating. The extraction and purification lines were conventional
in design being constructed of pyrex and employing all-metal bakeable valves
throughout. After two separate purifications with titanium sponge the samples were
admitted directly into the mass spectrometer. Nearly pure Ar38 (> 99.98%) was
used as a spike, this being metered into the system via a double tap reservoir system.
Calibration of the spike was against known amounts of atmospheric argon. From
the Table it can be seen that the values for argon concentrations of U. S. G. S. P-207
and M. I. T. B3203 agree with the accepted values within %; for the Bern muscovite
and biotite the measured values are within the range reported by other laboratories.
1
320
Table
J.
1.
W. Purdy
Analyses of standard minerals (values in 10
6
cm3 STP/g).
Standard sample
Reported value(s)
Measured with GD 150
U. S. G. S. P-207
M. I. T. B3202
Bern muscovite 4M
28.07»)
27.79, 28.08
387.7")
389.1, 392.1
5.75-6.61°)
6.23,6.27,6.43,
6.35, 6.35, 6.30,
6.38, 6.20
Bern biotite 4B
5.01-5.3Ie)
5.43, 5.35, 5.29,
5.30
") Lanphere, M. A. et al. (1967).
")HURLEY, P. M. et al. (1962).
c)Jäger, E. (1969).
The mean value for the Bern muscovite 4M is 6.31 x 10~6 cm3 STP with a standard
deviation of 0.5%. This is well within the scatter reported by other laboratories
(Jäger 1969) for this sample.
Conclusions
In terms of static sensitivity (6.10~*5 A cm 3 STP), absence of a background
correction at the critical mass 36 position, resolution of argon isotopes, reproducibil¬
ity of results, long-term stability of electronic components, lifetime of the filament
(in excess of one year when operated continuously), the Varian Mat GD150 is a
suitable instrument for measuring argon concentrations from terrestial samples. A
useful improvement for our instrument in terms of daily maintenance and running
costs would be the conversion of our present pumping system to an ion pumping
system.
Acknowledgments
The writer wishes to thank Professor E. Jäger for supplying laboratory space and facilities and
for much helpful advice during the course of this research. As well, help from other members of the
Bern Geochronology laboratory was appreciated. The Kanton of Bern financed the purchase of the
mass spectrometer; other aspects of the research were supported by the Schweizerischer National¬
fonds zur Förderung der wissenschaftlichen Forschung. Professor E. Jäger and Dr. A. E. Mussett
critically read the manuscript. The writer was financially supported by a Postdocturate Fellowship
from the National Research Council of Canada.
BIBLIOGRAPHY
Farrar,
E., Macintyre, R. M., York, D., and Kenyon, W. J. (1964): A Simple Mass Spectrometer
the
Analysis of Argon at Ultra-High Vacuum. Nature 204, 531.
for
P.
Hurley, M., Fairbairn, H. W., and Pinson, W. H., Jr., et al. (1962): Variations in Isotopie Abun¬
dances of Strontium, Calcium, and Argon and Related Topics. NYO-3943 Tenth Annual Progress
Report for 1962 to U. S. Atomic Energy Commission.
Jäger, E. (1969): Colloquium on the Geochronology of Phanerozic Orogenic Belts, programme (Bern
and Zurich, 23 August to 4 September 1969).
Lanphere, M. A., and Dalrymple, G. B. (1967): K-Ar and Rb-Sr Measurements on P-207, the
U.S.G.S. Interlaboratory Standard muscovite. Geochim. Cosmochim. Acta 31, 1091-1094.
Nier, A. O. (1947): A Mass Spectrometer for Isotope and Gas Analysis. Rev. Sci. Instr. 18, No. 6,
398-411.
Rex, D. C, and Dodson, M. H. (1970): Improved Resolution and Precision of Argon Analysis Using
a MS 10 Spectrometer. Eclogae geol. Helv. 63, 275-280.